1. Field of the Invention
The present invention relates to a process for producing an aromatic carboxylic acid by the liquid-phase oxidation of alkylbenzene using oxygen. More particularly, the invention relates to a process of producing an aromatic carboxylic acid without reducing the purity and yield of the product by recycling the reaction mother liquor, as it is, formed by separating the aromatic carboxylic acid from the liquid-phase oxidation reaction product.
2. Description of the Prior Art
The process of producing an aromatic carboxylic acid by subjecting an alkylbenzene such as p-xylene to a liquid-phase oxidation with a molecular oxygen-containing gas in a lower aliphatic carboxylic acid solvent in the presence of a catalyst comprising a heavy metal such as cobalt, manganese, etc., and bromine is known as the SD process as disclosed in U.S. Pat. No. 2,833,816, and has been widely practiced on an industrial scale.
The aromatic carboxylic acid which is the desired product of the reaction is recovered from the reaction mixture obtained in the liquid-phase oxidation reaction using an ordinary separation such as filtration, precipitation, etc., and, on the other hand, the reaction mother liquor left contains almost all of the catalyst, in particular, the heavy metal catalyst used in the reaction. In this case, because the heavy metal, such as cobalt, manganese, etc., is an expensive material, to practice the oxidation reaction economically it is necessary to reuse the heavy metal catalyst effectively. For this purpose it is desirable to recycle the reaction mother liquor to the reactor for reuse as the solvent and catalyst in the subsequent reaction. However, since the reaction mother liquor contains also a large amount of water by-produced in the oxidation reaction in addition to the catalyst and solvent and if the amount of water in the reaction system is over a definite value, the oxidation reaction is greatly hindered and recycling the reaction mother liquor as it is or without removing the water therefrom is unsuitable.
Therefore, various processes have been proposed for removing water from the reaction mother liquor for recycling the reaction mother liquor to the oxidation reaction system. For example, in the process described in Japanese Patent Publication No. 5861/'69, water is removed from the reaction mother liquor, which is formed by separating terephthalic acid from the oxidation reaction product, by evaporating or distilling the reaction mother liquor at temperatures lower than 130.degree.C. Also, in the process described in Japanese Patent Publication No. 41,339/'72, water is removed from the reaction mother liquor by distilling the reaction mother liquor in the presence of oxygen or an oxygen-containing gas. In these processes, water can be effectively removed from the reaction mother liquor. However, the reaction mother liquor obtained by separating an aromatic carboxylic acid from the oxidation reaction product contains, as is known, various organic impurities such as intermediate products and by-products from the oxidation reaction besides the solvent and water by-produced in the reaction. Hence, if the reaction mother liquor is heated to high temperatures for a long period of time to remove water therefrom by distillation as in the aforesaid conventional process, the organic impurities contained in the reaction mother liquor form polymers which are harmful to the oxidation reaction. These polymers gradually accumulate in the reaction system, while the reaction mother liquor is repeatedly used by recycling, reducing the quality of the aromatic carboxylic acid produced. Furthermore, when the reaction mother liquor is distilled by heating as stated above, harmful metallic impurities such as iron, etc., enter the reaction system due to the erosion of the distillation apparatus, etc., which also results in reducing the quality of the aromatic carboxylic acid. As described above, it is difficult in such conventional processes to avoid the accumulation of a large amount of harmful impurities in the reaction mother liquor recovered and thus the aforesaid conventional processes for removing water from the reaction mother liquor are not useful for recovering the reaction mother liquor for reuse.